1. Field of the Invention
This invention relates to a method and apparatus for measuring the force applied to a seat belt. Specifically, a sensor arrangement is mounted on a rigid plate secured between a seat belt portion and a vehicle structure to provide accurate seatbelt force measurements.
2. Related Art
Most vehicles include airbags and seatbelt restraint systems that work together to protect the driver and passengers from experiencing serious injuries due to high-speed collisions. It is important to control the deployment force of the airbags based on the size of the driver, or the passenger. When an adult is seated on the vehicle seat, the airbag should be deployed in a normal manner. If there is an infant seat secured to the vehicle seat then the airbag should not be deployed or should be deployed at a significantly lower deployment force. One way to control the airbag deployment is to monitor the weight of the seat occupant.
Current systems for measuring the weight of a seat occupant are complex and expensive. One type of system uses pressure sensitive foil mats mounted within the seat bottom foam. Another system uses sensors placed at a plurality of locations within the seat bottom. The combined output from the mats or the sensors is used to determine the weight of the seat occupant. If the sensors become damaged or fail to operate for some reason, the system will not provide accurate seat weight measurements and airbag deployment could occur under undesirable conditions.
Also mounting sensor systems within the seat can be difficult and time consuming. It is difficult to find mounting locations for each the sensors that will accommodate all of the various positions of a seated occupant while still providing accurate measurements. Further, shifting of the occupant on the seat can dislodge or move the sensors out of their proper location. Because the sensors are mounted within the seat bottom, it is difficult to reposition the sensors after the seat is installed in the vehicle.
Current sensor systems also can have difficulty determining whether an adult is belted to the seat or whether a child car seat is belted to the seat. When a child seat is secured to a seat with a seat belt, an excess force acts on the sensors mounted within the rear portion of the seat bottom, which interferes with accurate weight sensing. Over tightening of the seatbelt to securely hold the child seat in place, pulls the child seat down against the rear part of the seat bottom, causing the excessive force measured by the sensors. Due to this effect, the current weight sensing systems have difficulty in discerning between an adult belted to a seat and a child seat secured to the seat with a seat belt.
Thus, it is desirable to have a system for determining whether conditions are proper for deploying an airbag by determining whether a child seat or an adult is secured to the seat with a seat belt. The system should further work with traditional seat occupant weight sensing systems and should provide accurate measurements, be easy to install, and overcome the above referenced deficiencies with prior art systems.
A seat belt sensor system includes a load cell with a strain gage that is integrated into a seat belt mechanism that is used to secure an occupant to a vehicle seat. When the seat belt is tightened, the sensor is pulled into tension and this is measured by the strain gage. The strain gage measurements and signals are send to an electronics unit that processes the signals and feeds the signal back to an occupant sensing control unit. Occupant sensing control unit uses the information to determine whether a child seat or an adult is belted to the vehicle seat an ultimately controls the deployment of an airbag mechanism.
In a disclosed embodiment of this invention, the sensor assembly includes rigid member with a first end for supporting a seat belt portion and a second end for attachment to a vehicle structure, such as a B-pillar or seat mount, for example. The strain gage is mounted on the rigid member between the first and second ends and is used to measure the strain exerted on the rigid member by tension forces applied to the seat belt portion. An electrical connector is also mounted to the rigid member next to the strain gage. The electrical connector receives the strain measurements and transmits the measurements to a central processor to determine the magnitude of the tension force.
Preferably the rigid member is formed as a metallic plate that is defined by a length, width, and thickness. The length is greater than the width and the thickness is significantly less than the length and the width. The rigid member includes a neck portion positioned between the first and second ends with the width being less than the width of the first and second ends. The strain gage is mounted on the neck portion and measures the strain resulting from tension forces exerted on the first end of the rigid member by the seat belt.
In a preferred embodiment, the sensor assembly is incorporated into an occupant sensing control system that controls deployment of safety devices such as an airbag based on the tension forces measured in the seat belt. The strain gage generates a signal representative of the tension forces in the seat belt and transmits the signal to an electronic controller or processor. The airbag is prevented from deploying if the signal exceeds a predetermined limit.
A method for controlling airbag deployment includes the following steps. The seat belt assembly is provided with a buckle strap attached to a male buckle member and a seat belt latch mechanism with a female receptacle for receiving the male buckle member to secure the occupant to the vehicle seat. The rigid plate has one end secured to a portion of the seat belt and an opposite end of the plate is secured to a vehicle structure with a strain gage mounted to the rigid plate between the ends. The male buckle member is latched to the female receptacle and a tension force is generated on the seat belt assembly by tightening the buckle strap. The strain is measured on the rigid plate due to the tension force with the strain gage. A tension force signal is generated based on strain measurement and deployment of an airbag is controlled based on the tension force signal.
The airbag is not deployed if the tension in the seat belt exceeds a predetermined limit. By measuring the tension in the seat belt a differentiation can be made between an adult belted to a seat and a child seat belted to the seat. Thus, deployment of the airbag can be more effectively controlled and will not be deployed when a child seat is belted in place.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.